Many clinicians have applied orthodontic forces to successfully reposition teeth only to observe upon completion of treatment that one or more teeth have relapsed into their near original position. This problem is a clinical reality for which the underlying biological basis is not understood. We hypothesize that there are unique patterns of gene expression in the periodontal ligament (PDL) and underlying alveolar bone (AB) of teeth undergoing OTM and OMT-R. Using differential display, this study seeks to identify mRNA species for differentially expressed genes in the PDL and AB of teeth with and without OTM in addition to teeth that have had OTM-R in the Sprague-Dawley rat. Using reverse transcription polymerase chain reaction (RT-PCR) DNA sequences corresponding to these differentially expressed mRNAs will be identified through high resolution denaturing polyacrylamide gal electrophoresis. These sequences will then be compared, through the use of cDNA probes, to several known mediators of OTM and bone remodeling. These mediators relating to OTM are: IL-1 alpha and beta, IL-1 receptor, IL-2, IL-2 receptor, TGF-B, TGF-B receptor, PDGF, PDGF receptor, GM-CSF, GM-CSF receptor, PTH-rP, PTH-rP receptor, SP, SP receptor, VIP, VIP receptor, CGRP, and Osteopontin. Those mRNA species non-hybridizable to the above cDNA mediators will be cloned and sequenced using a 5' Rapid Amplification of cDNA Ends (5' RACE) approach. Once the mediators of OTM and OTM-R have been identified, their individual and/or interactive roles can be ascertained with the ideal of enhancing OTM while mitigating OTM-R. Key Words: Differential Display, mRNA, Orthodontic Tooth Movement, Relapse.